Determination of Silica Fume in Unhydrated, Blended, Dry-Packaged Mixture, and Hydrated Mortar

Abstract

Abstract A test method is proposed as a quality control methodology to determine the relative amount of dry silica fume present in a prepackaged blend of dry, unhydrated, cementitious material. The method assumes availability of representative samples of each of the raw material ingredients prior to packaging, including portland cement, silica fume, fly ash, silica sand, and a representative sample of the mixed product. Known percentages of silica fume by weight of portland cement were added separately to a basic blended mixture and analyzed by combined heat treatment and X-ray diffraction (XRD) method. From a calibration curve developed from standard laboratory mixtures, an estimate of the amount of silica fume in a prepackged cement was determined with a reasonable degree of accuracy. Additionally, a neat portland cement and portland cement + fly ash + sand mixtures containing different proportions of silica fume by weight of cement were hydrated at W-C = 0.30 and analyzed at selected periods for up to 180 days to determine residual silica fume. The diminution of calcium hydroxide (formed by the hydrating portland cement) in the cement-fly ash mixture and in the combined silica fume and fly ash cement mixtures were monitored during this period using thermogravimetric analysis and scanning electron microscopy.